A known type of robotic arm is used in path following or tip following applications. Such an arm may be caused to advance into an environment along a required path, for example avoiding obstacles, with the body of the arm remaining close to that path along its length. Such an arm commonly comprises a plurality of sequentially arranged articulated links. Actuators comprising control cables may be used to control the position of control links which are positioned at intervals along the length of the arm. For example, there may be three cables associated with each control link. The control links each define the end of a segment in which the remaining links are passive, and take up a continuously curved shape between adjacent control links. Thus the shape of the body of the arm can be controlled.
With such an arrangement, the actuators may also comprise a motor with an encoder to pull in or pay out each cable as required to control the position of the control link concerned. A computer control system may communicate with the encoders for this purpose. For example, the control cables may each be wound on to a spool. When first constructed, the arm needs to be initialised, with the encoders set to correspond to a known position of the arm. For this purpose, it may be possible to allow the arm to hang down vertically, in which case it may be assumed that the arm is straight. The control cables may then be wound to predefined tensions, and the motor encoders may be set to “zero” or “straight”. The arm may then be packaged for use, for example being wound onto a reel inside a deployment housing.
The process of “zeroing” the encoders may need to be repeated after a certain period of use to calibrate the arm because of hysteresis errors. However, it may be inconvenient or impossible to allow the arm to hang down or otherwise to constrain the arm into a straight position when it has been packaged for use. Thus it has previously been necessary for the arm to be taken out of service and removed from the housing in order to calibrate it.